The invention relates in general to electric switches of the mechanical type, and more particularly to a low cost, infinitely-variable, thermal cycling switch control for regulating the the heat output or wattage of a resistance type surface heating unit of a domestic electric cooking range.
A typical domestic cooking range of the electric type includes a horizontal cooking platform providing four surface heating units upon which cooking utensils, such as pots and pans, are placed for heating. Associated with each unit is a separate infinitely-variable control that maintains its associated surface heating unit at a wattage output determined by the position of a user-rotatable knob of the control. Such wattage output maintenance is effected by cycling the resistance type heating unit on and off at a predetermined rate, the control including a self-heated bimetallic switch element to automatically cause such thermal cycling. Movement of the thermal cycling switch control knob mechanically biases the bimetallic element by means of a disc cam to vary the percentage on time of the controlled heating unit to produce an average power consumption corresponding to the desired wattage.
Typically, four thermal cycling switch controls are mounted on the back of a vertically extending control panel provided across the rear edge of the horizontal cooking platform supporting the surface heating units. Rotatable shafts of the control units extend through the control panel. The user-accessible knobs are mounted on the distal ends of such shafts. Rotation of each knob sets the wattage (heat output)of its controlled heating unit at a point within the range of a no heat "off" condition to a high wattage (heat output) "on" condition. Rotational detent is typically provided at the "high" wattage (heat output) position to generate tactile feedback for the range user.
For purposes of safety, such controls also include a locking detent at the "off" position of the control to preclude the inadvertent turning on of its associated heating unit. Unlocking of the detent is provided by pushing in on the control knob, to axially shift the control shaft against the force of a biasing spring, wherein turning of the knob to electrically energize the associated heating unit is then possible.
Such push-to-turn thermal cycling switches or surface heating unit wattage controls are well known in the art, as represented by U.S. Pat. No. 3,932,830, assigned to the assignee of the present invention and incorporated by reference herein in its entirety. The locking "off" position detent means illustrated in the incorporated '830 patent includes a notched, washerlike member fixed to the axially shiftable spring-biased control shaft that engages with a locking tab provided by a switch cover. Accepted industry standards require that the locking "off" position detent means, such as the type illustrated in the '830 patent, withstand at least 30 in/lbs of torque and provide at least a 6000 cycle rotational wear life.
The push-to-turn control of the type illustrated by the '830 patent includes a helical compression spring for axially biasing the control shaft against the switch cover to ensure detent locking at the "off" position. Such a spring is placed under compression during assembly of the switch. Proper positioning of associated switch components must be maintained against forces generated by the spring undergoing compression during such assembly.